Whenever two current-conducting coils are placed in space so that the magnetic field generated by each of them is also linked to the other, electrical energy transfer can take place; such form of transfer is normally termed as Inductive Power Transfer (IPT).
Power transfer is in general easier to achieve when the coils are tightly coupled, that is when the magnetic flux generated by each coil almost completely links the other. The degree of coupling is often expressed in terms of the coupling coefficient k defined as:
                    k        =                  M                                                    L                1                            ·                              L                2                                                                        (        0.1        )            
where
k is the coupling coefficient and 0≤k≤1
L1 is the inductance of a first coil, and
L2 is the Inductance of a second coil.
M is the mutual inductance of the two coils.
The most widespread form of IPT is the classical transformer, where the two coils are wound around a common ferromagnetic core, resulting in the highest possible degree of coupling. In such structure, the coupling coefficient is typically very close to unity.
Loosely coupled circuits, on the other hand, are characterized by a coupling coefficient typically lower than 0.5, meaning that a significant part of the flux generated by each coil does not link the other, resulting in magnetic “leakage”. This is almost invariably the case when the coils are built on physically separated structures with non-negligible separation distance between them.
From the above description, it is also easy to understand that k is in general a function of the relative position of the two coils and may vary widely when either of the coils moves with respect to the other.
The problem of transferring power between two physically separated structures that can move with respect to each other can therefore be stated in terms of magnetic circuits as a problem of Inductive Power Transfer between loosely coupled coils with variable coupling coefficient.
In order to optimize the power transfer process and reduce losses, the physical principle of resonance can be applied to loosely-coupled magnetic circuits. A Capacitive component is added to both coils, so that the reactive power consumed by the leakage inductance is compensated by the added capacitance; such perfect compensation is achieved at the resonant frequency:
                                                        f                              0                ,                send                                      =                          1                              2                ⁢                                                                  ⁢                π                ⁢                                                                            L                      1                                        ⁢                                          C                      1                                                                                                    ;                ⁢                                  ⁢                              f                          0              ,              pick                                =                      1                          2              ⁢                                                          ⁢              π              ⁢                                                                    L                    2                                    ⁢                                      C                    2                                                                                                          (        0.2        )            
Most state-of-the-art IPT systems or apparatuses are designed so that the resonant frequencies of both coils coincide, and power transfer is performed by exciting the transmitting coil exactly at such common resonant frequency, achieving very good transfer efficiency and minimized Volt-Ampere ratings at a specific operating conditions of the sending and pickup converters that are operated at unity power factor, as the reactive power is completely contained within the resonant tanks constituted by the coils and the compensating capacitors.
In the French patent publication Fr 3004596 it is disclosed a method for inductively charging a vehicle. The primary and secondary coils of the charging system is mutually movable.